24.2.4 Asynchronous I/O Signals

The signals listed in this section are used in conjunction with
asynchronous I/O facilities. You have to take explicit action by
calling fcntl to enable a particular file descriptor to generate
these signals (see Interrupt Input). The default action for these
signals is to ignore them.

Macro: intSIGIO

This signal is sent when a file descriptor is ready to perform input
or output.

On most operating systems, terminals and sockets are the only kinds of
files that can generate SIGIO; other kinds, including ordinary
files, never generate SIGIO even if you ask them to.

On GNU systems SIGIO will always be generated properly
if you successfully set asynchronous mode with fcntl.

Macro: intSIGURG

This signal is sent when “urgent” or out-of-band data arrives on a
socket. See Out-of-Band Data.

Macro: intSIGPOLL

This is a System V signal name, more or less similar to SIGIO.
It is defined only for compatibility.